The present invention relates to process control systems. More specifically, the present invention relates to isolation circuitry for use in transmitters of a process control system
Process control systems are used in manufacturing plants to monitor operation of a process. A transmitter is placed in the field and monitors a variable of the process, for example, pressure, temperature or flow. The transmitter couples to a control loop and transmits information over the control loop to a controller which monitors operation of the process. Typically, the control loop is a two-wire loop carrying a current which also provides power for operation of the transmitter. Communication standards include the Fieldbus standard in which digital information is sent to the transmitter. HART.RTM. is another standard which allows communication over a 4-20 mA process variable signal.
One type of process variable sensor is a resistance bridge circuit in which the resistance of the bridge varies in response to the process variable. Other sensors include capacitance, vibrating beam, or other. An input signal is applied to the bridge and the bridge output is monitored to determine the process variable. To meet certain Intrinsic Safety standards, the bridge circuit must be "infallibly" electrically isolated from the rest of the transmitter. Such standards are set forth by, for example, European CENELEC standards EN50014 and 50020, Factory Mutual Standard FM3610, the Canadian Standard Association, the British Approval Service for Electrical Equipment in Flammable Atmospheres, the Japanese Industrial Standard, and the Standards Association of Australia. The Intrinsic Safety requirements are intended to guarantee that instrument operation or failure cannot cause ignition if the instrument is properly installed in an environment that contains explosive gasses. This is accomplished by limiting the maximum energy stored in the transmitter in a worst case failure situation. Excessive energy discharge may lead to sparking or excessive heat which could ignite an explosive environment in which the transmitter may be operating.
The prior art has primarily used two techniques to achieve infallible isolation between the sensor circuitry and the transmitter circuitry. The first technique is to provide sufficient mechanical segregation or spacing in the sensor such that it is impossible for a component failure to cause electrical shorting to another component or ground. The second technique is to design the entire system such that isolation is not required by using components which are rated for large power dissipation such that they themselves are considered infallible.
One problem with both of these techniques is that they require a sufficiently large transmitter housing to provide the required spacing between components or the relatively large size of the high power components. Thus, reduction in transmitter size has been limited when complying with Intrinsic Safety requirements using the above two techniques.